Before the imposition of stringent efficiency standards by the government, furnaces typically operated with relatively high excess air and high flue temperatures and at relatively low efficiencies. Many of such prior art furnaces used natural draft convection to exhaust products of combustion from the furnace. Under these conditions, condensation was unlikely to result in any appreciable quantities from the products of combustion, even during cold start-up, low fire operation. In addition to the disadvantage of relatively low fuel economy, such furnaces had low discharge temperatures, thereby detracting from space comfort, particularly during low fire, cold start operation.
Higher efficiency furnaces more commonplace today typically operate with lower flue temperatures and less excess air. Products of combustion are exhausted by forced draft blowers, rather than by natural convection. As a result of these design changes, condensation is more likely to occur in the furnace, particularly during low fire, cold start operation.
One approach known in the art for counteracting condensate formation during cold start operation in a multi-stage or variable input furnace is to operate the furnace initially in a high fire mode for a fixed amount of time. Upon expiration of the fixed amount of time, the furnace is operable at either low fire or high fire, depending upon the demand for heating. This type of approach is described in U.S. Pat. Nos. 4,425,930; 4,976,459; and 4,982,721. This approach is somewhat effective in reducing condensation during furnace start-up. However, the fixed amount of time may be more or less than what is actually needed to adequately warm up the furnace and to inhibit condensate formation.
There is, therefore, a need for improved apparatus and method for controlling furnace combustion during cold start-up, in order to reduce condensate formation and enhance space comfort.